1. Field of the Invention.
This invention relates to synchronizing operation of stations in separate control networks and in particular to a system in which an interface station in one network communicates synchronization data over a data link to stations in another network.
2. Description of the Prior Art.
A distributed control system typically consists of several Local Area Networks (LANs) interfaced via point-to-point data links. Each LAN typically comprises a plurality of stations such a microcomputers, controllers, loggers, visual displays or printers that are connected to a communication medium, such as a bus, which is common to all of the devices in the LAN. The common communication medium can take any one of various forms, including a parallel or serial bus over which individual stations can be addressed for communication. Some stations on the LAN are active in the sense that they can initiate transmissions on the bus. Other stations are passive in that they only respond when addressed by another device on the bus. The point-to-point data links used to interface the LAN's can take many different forms, including electrical cabling, optical fibers, microwave channels, or phone lines. The point-to-point links can also include devices such as low speed MODEMS and can introduce time delay in the communication link.
In distributed control systems, it is useful to achieve system synchronization, that is, some stations in different LAN's must be timed to work with one another, or have the same time reference. Such synchronization is crucial in automatic control systems to ensure the correct timing of events, and is crucial in communication systems to ensure efficient use of a communication link.